Well, they claim better specificity and safety than regular antisense (tho they don't mention PMO).
This article from August 2017 lays it all out.
http://www.nature.com/articles/nbt.3948 They say regular oligoes have millions of molecules that are just useless because only a small proportion end up with the "right" atomic positioning (up or down).
I certainly can't know if any of this stuff really matters that much but they certainly think it does.
Would be nice if an outsider has commented on how significant an improvement this stereopure molecule really is. Maybe no one can tell until we see how it comes out in the clinic.